Sorry to be vague, but there are so many variables to consider before properly answering that question. Here’s a few off the top of my head:
-which exercise are you doing?
-why are you doing that exercise?
-does that choice align with your client’s goals?
-who is your client? i.e., age; goals/needs; health/medical situation; time, energy, financial budget; level of fitness/exercise experience etc…
-how many other exercises are you doing?
-is there rest in between sets?
-what does current research suggest re: number of sets, based on specific goals/circumstances?